Method and composition for corrosion inhibition of carbon steel by ammonium sulfate

ABSTRACT

Corrosion of carbon steel storage and mixing equipment is inhibited by admixing an effective amount of a member selected from the group consisting of ammonium polyphosphate, sodium nitrite, potassium nitrite, ammonium thiosulfate, sodium thiosulfate and potassium thiosulfate with an aqueous solution of ammonium sulfate useful as a fertilizer.

DESCRIPTION BACKGROUND OF THE INVENTION

This invention relates to a method and composition for inhibition ofcorrosion of carbon steel storage and mixing tanks containing aqueoussolutions of ammonium sulfate by addition thereto of small amounts ofammonium polyphosphate, sodium nitrite, potassium nitrite, ammoniumthiosulfate, sodium thiosulfate or potassium thiosulfate. Ammoniumsulfate, in the form of aqueous solution is useful as a fertilizer. Theequipment employed in mixing and storing these aqueous fertilizersolutions is commonly constructed of carbon steel. However, aqueousammonium sulfate solutions are acidic and as such, corrosive toequipment made of carbon steel. If such corrosion proceeds unchecked,damage to equipment, contamination and loss of aqueous ammonium sulfatesolution can become substantial, especially at periods of peakfertilizer demand.

U.S. Pat. No. 3,930,832 discloses that corrosion of carbon steel by anaqueous solution of zinc nitrate, ammonium nitrate and urea is inhibitedby incorporating therein an effective amount of ammonium thiosulfate.

U.S. Pat. No. 4,191,550 discloses that incorporation of a water-solublethiosulfate into storage-stable liquid compositions of magnaesiumnitrate, ammonium nitrate and urea satisfactorily provides protectionfor carbon steel equipment without deleterious effect on the stabilityor salt-out temperature of the composition.

Accordingly it is one object of this invention to provide a simplemethod and composition for corrosion inhibition of carbon steelequipment in contact with a aqueous ammonium sulfate solution.

SUMMARY OF THE INVENTION

In accordance with the present invention there is provided an aqueousammonium sulfate fertilizer solution, the improvement which comprises aneffective amount of a carbon steel corrosion inhibitor selected from thegroup consisting of ammonium polyphosphate, ammonium thiosulfate, sodiumthiosulfate, potassium thiosulfate, potassium nitrite and sodiumnitrite.

The present invention also provides a fertilizer composition comprisingammonium sulfate and an effective amount of a carbon steel corrosioninhibitor selected from the group consisting of ammonium polyphosphate,ammonium thiosulfate, sodium thiosulfate, potassium thiosulfate,potassium nitrite and sodium nitrite.

There is further provided by the present invention a process forinhibiting carbon steel corrosion by an aqueous ammonium sulfatefertilizer solution comprising adding to said solution a member selectedfrom the group consisting of ammonium polyphosphate, sodium nitrite,potassium nitrite, ammonium thiosulfate, sodium thiosulfate andpotassium thiosulfate in an amount effective to substantially inhibitcorrosion of carbon steel in contact with said solution.

DETAILED DESCRIPTION

The present invention provides a method, solution and composition forinhibiting corrosion of carbon steel storage and mixing equipment forholding aqueous ammonium sulfate fertilizer solution by admixing aneffective amount, usually at least about 0.1% by weight of a memberselected from the group consisting of ammonium polyphosphate, ammoniumthiosulfate, sodium thiosulfate, potassium thiosulfate, potassiumnitrite and sodium nitrite. An upper limit on the effective amount of acarbon steel corrosion inhibitor has not been found to be critical; anamount of one of the carbon steel corrosion inhibitors of the presentinvention in excess of at least about 0.1% by weight has been found tobe effective. Only economic consideration would preclude employing aneffective amount of one of these corrosion inhibitors in excess of 5% byweight. By operating in accordance with the present invention, corrosionof carbon steel equipment used in mixing and storing aqueous ammoniumsulfate solution is substantially inhibited when the ammonium sulfatefertilizer composition comprises at least about 0.1% by weight of amember selected from the group consisting of ammonium polyphosphate,ammonium thiosulfate, sodium thiosulfate, potassium thiosulfate, sodiumnitrite and potassium nitrite. In preferred embodiments of the presentinvention the inhibitor of carbon steel corrosion by aqueous ammoniumsulfate solution exceeded 90%, compared to uninhibited solutions, whenthe ammonium sulfate fertilizer composition comprised 0.5% by weight ofammonium polyphosphate or 0.5% by weight of sodium nitrite. In anotherpreferred embodiment, the inhibition of corrosion of the carbon steel isessentially 100% when the aqueous ammonium sulfate solution comprised0.6% by weight ammonium thiosulfate. While the effective amount of oneof the corrosion inhibitors disclosed herein can exceed at least about0.1% by weight, it is one of the features of the present invention thatonly a relatively small amount, i.e., usually from at least about 0.1 toabout 5% by weight of one of the corrosion inhibitors recited above isnecessary for almost complete inhibition of carbon steel corrosion byaqueous ammonium sulfate solutions.

The ammonium sulfate fertilizer compositions of the present inventionare prepared by admixing an effective amount, usually from at leastabout 0.1 to about 5.0% by weight of a carbon steel corrosion inhibitorselected from the group consisting of ammonium polyphosphate, ammoniumthiosulfate, sodium thiosulfate, potassium thiosulfate, potassiumnitrite and sodium nitrite in a minimum amount of water with an aqueousammonium sulfate solution. While the inhibition of corrosion of thecarbon steel equipment is maximized when an effective amount of acorrosion inhibitor preferably in the form of an aqueous solution, isadded concurrent with the dissolution of the ammonium sulfate, inaccordance with the present invention, further corrosion of carbon steelalready in contact with an aqueous solution of ammonium sulfate is alsoeffectively retarded by addition to the standing solution of aneffective amount, usually about at least about 0.1 to about 5.0% byweight of a carbon steel corrosion inhibitor selected from the groupconsisting of ammonium polyphosphate, ammonium thiosulfate, sodiumthiosulfate, potassium thiosulfate, potassium nitrite and sodiumnitrite.

While the prior art teaches that special precautions, i.e. equipmentconstructed of special alloys or equipped with glass liners werenecessary to inhibit the corrosion of carbon steel by ammonium sulfatefertilizer solutions, the present invention provides a simple,inexpensive fertilizer composition solution, and method for inhibitingcorrosion of carbon steels, for example, 7-283 and 7-285 steels, whichsteels are commonly used for equipment to mix, process and storeammonium sulfate fertilizer solutions.

Ammonium sulfate, an article of commerce, is produced as a by-product inthe manufacture of caprolactam as well as by direct synthesis. Solidammonium sulfate is highly soluble in water; aqueous solution containingup to about 41.4% by weight ammonium sulfate can be prepared at 0° C.Aqueous solutions of ammonium sulfate, useful as a fertilizer, vary incomposition from 30 to 44% by weight, typically 38-42% by weight ofammonium sulfate. It is to be understood, however, that theconcentration of ammonium sulfate is not critical to the effectivenessof the corrosion inhibitors of the present invention.

Whereas in the past handling and storage of uninhibited ammonium sulfatesolution, at the height of the growing season, resulted in severecorrosion of carbon steel equipment which corrosion both consumed andcontaminated the fertilizer solutions, the present invention provides amethod, an aqueous ammonium sulfate fertilizer solution and compositionwhich almost completely eliminates the corrosion of carbon steel storagetanks, mixing equipment and tank trucks used in preparing, shipping andapplying aqueous ammonium sulfate fertilizer solutions.

While aqueous ammonium sulfate fertilizer solution comprising aneffective amount of a carbon steel corrosion inhibitor have beendiscussed, the present invention also contemplates a fertilizercomposition comprising ammonium sulfate and an effective amount of acarbon steel corrosion inhibitor selected from the group consisting ofammonium polyphosphate, ammonium thiosulfate, sodium thiosulfate,potassium thiosulfate, potassium nitrite and sodium nitrite. The amountof a carbon steel corrosion inhibitor of the present invention foundeffective in accordance with the present invention is at least about0.1% by weight of the composition. The fertilizer compositions of thepresent invention can be prepared by any convenient means, for exampleby directly admixing an effective amount one of the carbon steelcorrosion inhibitors of the present invention, in solid form with solidammonium sulfate or alternatively by adding an effective amount of acorrosion inhibitor to a step or solution in the preparation orpurification of ammonium sulfate.

Table I, below, summarizes corrosion inhibition of coupons of carbonsteel 7-285 immersed in 38% by weight aqueous soluions of ammoniumsulfate at room temperature (25° C.) for 10-20 days when an effectiveamount of ammonium polyphosphate, sodium nitrite or ammonium thiosulfatewas admixed during the dissolution of the ammonium sulfate. The rate ofcorrosion of a coupon of carbon steel 7-285, immersed in a 38% by weightaqueous ammonium sulfate solution without one of the corrosioninhibitors of the present invention was 17.0 mils per year. When one ofthe preferred carbon steel corrosion inhibitors of the present inventionwas admixed during dissolution of the ammonium sulfate, the percent ofinhibition of carbon steel corrosion ranged from more than 90% up to98%, compared to carbon steel in contact with uninhibited ammoniumsulfate.

                  TABLE I                                                         ______________________________________                                        CORROSION INHIBITION OF CARBON STEEL.sup.1                                    CONTAINING 38% BY WEIGHT OF                                                   AMMONIUM SULFATE IN WATER                                                                           Rate of   Percent                                               Inhibitor.sup.2                                                                             Corrosion.sup.3                                                                         Corrosion                                     Run     (% by weight) (mils/yr) Inhibition.sup.5                              ______________________________________                                        1       None          17.0      --                                            2       0.5% ammonium 1.6.sup.4 90.6                                                  polyphosphate                                                         3       0.5% sodium   1.0       94.1                                                  nitrite                                                               4       0.6% ammonium 0.35      98                                                    thiosulfate                                                           ______________________________________                                         .sup.1 Carbon steel 7285                                                      .sup.2 Admixed during dissolution of ammonium sulfate                         .sup.3 After 10 days at room temperature = 25° C.                      .sup.4 After 20 days at room temperature = 25° C.                      .sup.5 Compared to uninhibited ammonium sulfate solutions                

While the results of carbon steel corrosion inhibition summarized inTable I illustrates preferred embodiments of the present inventionwherein aqueous solutions of preferred corrosion inhibitors are admixedwith the ammonium sulfate solution during dissolution, addition of aneffective amount, usually from about 0.1 to about 5.0% by weight of thecorrosion inhibitors selected from the group consisting of ammoniumpolyphosphate, ammonium thiosulfate, sodium thiosulfate, potassiumthiosulfate, sodium nitrite and potassium nitrite to previously preparedammonium sulfate fertilizer solutions is also contemplated within thescope of the present invention.

EXPERIMENTAL Example 1

A coupon of carbon steel 7-285, having a total surface area of 36.315square centimeters and weighing 69.6972 grams was immersed in a 38% byweight aqueous solution of ammonium sulfate (8-0-0-9(S) gradefertilizer) for a period of 10 days. The weight loss by the coupon was0.3346 grams; this is equivalent to a corrosion rate of 17.0 mils peryear for carbon steel 7-285.

Example 2

A coupon of carbon steel 7-285, having a total surface area of 37.069square centimeters and weighing 73.2348 grams was immersed in a 38% byweight aqueous solution of ammonium sulfate (8-0-0-9(s) gradefertilizer) inhibited with 0.5 percent by weight sodium nitrite (NaNO₂)for a period of ten days. The weight loss by the coupon was 0.0225grams; this is equivalent to a corrosion rate of one mil per year.

Example 3

A coupon of carbon steel 7-285, having a total surface area of 36.309square centimeters and weighing 71.3065 grams was immersed in a 38percent solution of ammonium sulfate (fertilizer grade: 8-0-0-9S)inhibited with 0.6 percent by weight ammonium thiosulfate and held atroom temperature for ten days. The measured weight loss was 0.0069grams, which is equivalent to a corrosion rate of 0.35 mils per year.

Example 4

A coupon of carbon steel 7-285 having a total surface area of 36.26square centimeters and weighing 62.2165 grams was immersed in a 38percent solution of ammonium sulfate (fertilizer grade: 8-0-0-9S)inhibited with 0.5 weight percent (equivalent to about 0.17 weightpercent P₂ O₅) ammonium polyphosphate solution (Poly-N 10-34-0fertilizer grade: 60 percent of the phosphate as polyphosphate) and heldat room temperature for twenty-eight days with mild agitation. Themeasured weight loss was 0.0913 grams, which is equivalent to acorrosion rate of 1.6 mils per year.

Example 5

A coupon of carbon steel 7-285 is immersed in a 38 weight % aqueoussolution of ammonium sulfate containing 0.1 weight % potassium nitriteat 25° C. for a period of 20 days. The inhibition of carbon steelcorrosion is similar to that in Example 2.

Example 6

A coupon of carbon steel 7-285 is immersed in a 38% by weight aqueoussolution of ammonium sulfate containing 0.1% by weight sodiumthiosulfate at 25° C. for 20 days. The inhibition of corrosion of thecarbon steel is similar to that of Example 3.

Example 7

A coupon of carbon steel 7-285 is immersed in a 38% by weight aqueoussolution of ammonium sulfate containing 0.1% by weight potassiumthiosulfate at 25° C. for a period of 20 days. The inhibition ofcorrosion of carbon steel is similar to that recited in Example 4.

Example 8

A coupon of carbon steel 7-283 is immersed in a 38% by weight aqueoussolution of ammonium sulfate containing 0.1% by weight of ammoniumthiosulfate at 25° C. for 20 days. The inhibition of corrosion for the7-283 carbon steel is similar to that recited in Example 4.

What is claimed:
 1. In an aqueous ammonium sulfate fertilizer solutionin contact with carbon steel, the improvement which comprises at leastabout 0.1 to about 5% by weight of a carbon steel corrosion inhibitorselected from the group consisting of ammonium polyphosphate, ammoniumthiosulfate, sodium thiosulfate, potassium thiosulfate, potassiumnitrite and sodium nitrite.
 2. A fertilizer composition in contact withcarbon steel comprising ammonium sulfate and at least about 0.1 to about5% by weight of a carbon steel corrosion inhibitor selected from thegroup consisting of ammonium polyphosphate, ammonium thiosulfate, sodiumthiosulfate, potassium thiosulfate, potassium nitrite, and sodiumnitrite.
 3. The fertilizer composition described in claim 2 wherein thecomposition contains from about 0.1 to about 5.0% by weight of ammoniumthiosulfate.
 4. The fertilizer composition described in claim 2 whereinthe composition contains from about 0.1 to about 5.0% by weight ofsodium nitrite.
 5. A process for inhibiting carbon steel corrosion by anaqueous ammonium sulfate fertilizer solution comprising adding to saidsolution from about 0.1% to about 5.0% ammonium polyphosphate, sodiumnitrite or potassium nitrite.
 6. The process as described in claim 5wherein from about 0.1% to about 5.0% by weight of sodium nitrite isadded.
 7. The process as described in claim 5 wherein from about 0.1% toabout 5.0% by weight of potassium nitrite is added.
 8. The process asdescribed in claim 5 wherein from about 0.1% to about 0.5% by weightammonium polyphosphate is added.